Accurate noise floor measurements.

The coherent graph contains three dB meters.
The upper one gives the largest value since the signal
was selected.
The middle one gives the average peak amplitude and is
useful to determine the amplitude of a CW signal from
its key down periods to get a value that is independent
of whether dots or dashes are received.
The lower value is average power.

The dB values are calculated from the baseband signal.
To measure dynamic ranges, the second fft must be disabled.
Otherwise strong signals will be attenuated by the selective
limiter that is associated with the second fft and the
noise blanker.

To get an accurate measurement for the noise floor,
the baseband filter is made rectangular with a bandwidth of
1kHz.
Figure 1 shows the screen when a pure X-tal oscillator at
10.7MHz is connected to the 10.7MHz IF input.

Fig.1.
10.7MHz spectrum at 3Hz resolution.
The same signal is fed to both inputs.
The green dots is the sum of the two channels while
the magenta dots shows the difference between the channels.

By feeding the same signal to both inputs the noise from the
A/D converter is reduced by 3dB while the hardware sideband
noise is unaffected. This is because all "signals", the
carrier and any modulation on it will be in phase in both
channels while A/D converter noise is uncorrelated.

From the table we see that the sideband noise is below 148dBc/Hz at
a frequency separation of 10kHz.
The noise floor contains only a small fraction of sideband noise
since the noise floor only goes down by 0.7dB (18%) when the strong
signal is switched off.
The sideband noise is therefore somewhere around -155dBc/Hz, an
excellent performance obtained by not using any variable frequency
oscillator.

Shape of noise sidebands

Figure 2 shows the same strong signal as figure 1, but in the
main spectrum. The bandwidth is about 20Hz but the window
function used for the fft is sine to the 9th power.
The baseband spectrum always uses a sine squared window.

Fig.2.
10.7MHz spectrum at 20Hz resolution with sine to power 9 used
as window function.
The signal is the same as in figure 1.

The 50Hz sidebands due to hum modulation somewhere, probably
in the test oscillator are suppressed by about 95dB.
The sideband noise is equal to the A/D noise floor at a
frequency separation of 700Hz.

One or two channels

Linrad is designed for use with two antennas simultaneously.
On VHF the optimum configuration is a cross yagi array in
the "X-configuration" which means that the two orthogonal
antennas are in 45 degrees vith respect to a horisontal plane.

For HF bands any combination of two antennas will give
an improvement but without any practical experience I can
not estimate by how much.

If two channels are used and the offending signal is equally
strong in both channels as would be the case when receiving
a horisontally polarised signal on a cross yagi in the
X-configuration, the noise floor at 10kHz separation will
be at -148dBc/Hz.
Converted to BDR at 3kHz bandwidth (3dB desensitation) this
number corresponds to 116dB - and that is when the desired
signal has the same polarisation as the offending signal.

In case the offending signal is in one channel only, as would be
the case for a cross yagi in the "+" configuration, the blocking
dynamic range is 3dB lower, 113dB.